1. Field of the Invention
The invention relates to a component of an optical circuit which is based on an optical waveguide used in optical communication, an optical sensor or the like, and also to a method of producing the component.
2. Related Art of the Invention
In the fields of optical communication and an optical sensor, research and development of an optical circuit having various functions are vigorously conducted in order to realize sophisticated optical signal processing or sensing. An optical circuit uses an optical waveguide through which light passes, as a fundamental element. In an optical waveguide, a core region having a higher refractive index is disposed in a clad having a low refractive index, thereby confining light into the core region so as to propagate therethrough. When a core is patterned, various functions can be realized. In the specification, a configuration in which an optical waveguide is patterned in a manner like an electric circuit is defined as an optical circuit.
Hereinafter, an example of an optical circuit used in optical communication will be described in detail.
FIG. 10 is a schematic section view of a usual quartz single-mode optical waveguide. A core 101 has a square section shape having a side of 8 xcexcm, and is covered by a quartz clad 102. Light propagates in the direction of the arrow X.
In FIG. 11, (a) to (c) show a method of producing an optical waveguide which is used most commonly in the prior art (for example, Kawachi, xe2x80x9cOptoelectronicsxe2x80x9d No. 8, p. 85, 1988). The production process includes the following steps.
(a) A core film 112 made of SiO2 doped with, for example, Ge is formed on the surface of a quartz substrate 111 serving also as a lower clad layer, by the flame deposition method (FIG. 11(a)). When a substrate other than a quartz substrate is used, a lower clad layer is previously formed on the substrate by the flame deposition method, and the core film 112 is then formed on the layer.
(b) The core film 112 is patterned into a predetermined pattern by using the photolithography or dry etching technique, thereby forming a core portion 112a (FIG. 11(b)).
(c) Finally, an upper clad layer 113 is formed by the flame deposition method to cover the core portion 112a (FIG. 11(c)).
According to this method, an optical waveguide of a low loss can be produced so that a complicated optical circuit is realized. Also methods in which the CVD method or the vapor deposition method is used as a film deposition method are under investigation.
Recently, such an optical waveguide device is more strongly requested to be produced at a low cost and in mass production.
In the field of optical communication, as typically exemplified by FTTH (Fiber To The Home), an optical fiber is being extended from a trunk line to a subscriber""s line. Therefore, it is required to produce a photoelectric conversion module which is based on an optical waveguide, in mass and at a low cost.
The prior art method of producing an optical waveguide such as shown in FIGS. 11(a) to 11(c) has an advantage that even a complicated optical circuit of high performance can be produced. However, a long tact time is necessary for the film forming and heating processes because the core and the clad are produced by a thin film forming process. Furthermore, the photolithography or dry etching technique which is used in the patterning of the core requires many complicated steps. Consequently, although the method is suitable for production of an optical circuit of high performance and high added value, such as an array waveguide grating, it cannot say that the method is suitable for production of a simple optical circuit such as a Y-branch splitter.
In order to solve the problems in production of an optical waveguide, various ideas have been proposed. For example, one of potential processes is press molding. Press molding is proposed in Japanese Patent Publication (Kokai) Nos. HEI8-320420 and 1-26806, etc. As shown in FIGS. 12(a) to 12(c), a desired core pattern 120a is formed in a die 120, and the die is pressed against a base material (a first glass substrate 121) serving also as a lower clad at a high temperature (FIG. 12(a)). The first glass substrate 121 is taken out from the die 120, an ultraviolet curing resin 123 is applied to the surface and filled into a core portion 123a(FIG. 12(b)), and a second glass substrate 122 is stuck to the first glass substrate and the resin is then irradiated with ultraviolet rays to be cured. Thus core pattern grooves are formed in one operation (FIG. 12(c)). According to this method, a mass production is enabled, and the photolithography or dry etching step which is used in the prior art can be omitted, and a core can be easily formed by filling a resin. Therefore, this is seemed to be a hopeful process of producing an optical waveguide.
According to study of the inventors, however, it has been proved that the method of FIGS. 12(a) to 12(c) has the following drawbacks. When a glass material is subjected to press molding by using a die having a core pattern of a square section which has a side of about 8 xcexcm, large concentrated stress is applied to the core pattern portion because the aspect ratio of the vertical and lateral sides of the section of the core groove is large. Consequently, the die is easily broken, so that the die must be frequently replaced with a new one. When a glass material having a coefficient of thermal expansion which is largely different from that of a die is subjected to press molding, large thermal stress acts so as to tighten the pattern in the die, thereby lowering the release property between the die and the glass material. As a result, it is difficult to realize a satisfactory pattern transfer.
Furthermore, the production of a die used in press molding has the following problem.
Conventionally, a glass lens is known as an example of an optical component which is produced by using press molding. In a die for forming a glass lens, a hard alloy such as WC is mainly used as the base material, and a desired surface pattern is obtained by means of machining such as cutting or polishing.
As described above, a core of an optical waveguide has a rectangular section shape having a side of about 8 xcexcm. It is very difficult to produce such a shape by means of machining, because a cutting tool must usually have a radius diameter of 10 xcexcm or more. In machining, a linear core pattern can be formed, but it is very difficult to realize a branched pattern or a smoothly bent pattern.
On the other hand, etching is known as a production method other than machining. In etching, a complicated core pattern can be obtained, but, usually, a material of high mechanical strength is hardly etched. A hard alloy is not an exception to the above. When etching which is deeper than 5 xcexcm is to be performed, an etching mask must have a very complicated configuration. Furthermore, there is another problem in that the surface is roughened as a result of etching, and, even when the surface is smoothed by polishing, an edge of a section of the pattern is rounded, with a result that an optical waveguide of high performance can be hardly obtained.
It is an object of the invention to provide an optical waveguide component which can be produced by a molding method of high productivity and without applying a large burden to a die, and can be accurately positioned with respect to an optical fiber or an active device in an easy manner and in mass, and a method of producing the optical waveguide component.
In the optical waveguide component of the invention, first and second optical members are stuck together to form an optical waveguide, and the optical members can be used as a platform (substrate) on which a device is to be mounted. In other words, a stage on which a light emitting device and a light receiving device are to be mounted are disposed in one of the first and second optical members. When sufficient relative positional accuracy is attained between the optical waveguide and the stage, positioning between the optical waveguide and the light emitting device or the light receiving device can be correctly performed. As a result, an optical transmission/reception module of a low optical loss can be realized.
In the optical waveguide component, the optical members are preferably made of glass. In this example, since a glass material is excellent in environmental resistance, heat resistance, and mechanical strength, it is possible to realize an optical waveguide component of high reliability.
In the optical waveguide component, preferably, the first and second optical members are configured by a combination of materials which are different in coefficient of thermal expansion from each other by 30xc3x9710xe2x88x927/xc2x0 C. or more. In this example, when the ambient temperature is changed, internal stress is generated owing to the difference in coefficient of thermal expansion between the members, and birefringence is induced in the optical waveguide, with the result that the polarization state of light propagating through the optical waveguide is sensitively changed. When the polarization state of light is observed, therefore, the optical waveguide component can be used as a temperature sensor.
In the optical waveguide component, preferably, a face of the optical member in which a core pattern groove is formed is not perpendicular to a side face in an end portion of the core pattern groove. According to this configuration, the direction of the light guidance by an optical fiber is made coincident with that of the optical waveguide. Even in the case where, when light enters the optical waveguide, light is reflected by the side face, therefore, it is possible to prevent the reflected light from returning toward the optical fiber. As a result, a signal noise is prevented from occurring.
In the optical waveguide component, the optical members are preferably provided with a positioning marker. In this example, grooves of the optical waveguide can be accurately positioned by making the markers respectively formed on the members, coincident with each other.
In the optical waveguide component, preferably, a recess is formed in one of the first and second optical members, and a projection having the same pattern as the recess is formed on the other member. In this example, when the recess and the projection respectively disposed in the members are fitted with each other, grooves of the optical waveguide can be accurately positioned.
In the optical waveguide component, preferably, section shapes of grooves respectively formed in the first and second optical members are symmetrical about the sticking face. Particularly, the optical waveguide has a circular or square section shape when the groove patterns are vertically combined with each other. In this case, a groove of one side has a section shape of a semicircle or a rectangular equilateral triangle. The reason of the above is as follows. In order to eliminate a difference in polarization property among waveguides, a core having a symmetric section shape is suitable. From the viewpoint of press molding, it is preferable to form a section pattern in which concentrated stress is not applied to a fine shape portion of a die.
A first method of producing an optical waveguide component according to the invention is characterized in that a predetermined core pattern groove is formed in surfaces of first and second optical members by means of press molding, a material of a high refractive index is sandwiched between the first and second optical members, and the material of a high refractive index is cured to be formed into a core layer. Since a core pattern groove is formed by a molding process, substrates having grooves of the same pattern can be produced in mass by a die. This is advantageous in production cost.
A second method of producing an optical waveguide component according to the invention is characterized in that a predetermined core pattern groove is formed in surfaces of first and second optical members by means of press molding, the first and second optical members are directly bonded to each other, and a material of a high refractive index is then filled into a cavity of the core pattern by means of capillarity. When such a method is employed, no air bubble is formed in the core, and hence an optical waveguide component of high performance can be produced. When a liquid is used as a core material and the end portions are sealed, the liquid can be easily filled into the core.
In the first production method, preferably, the material of a high refractive index is at least one resin selected from the group consisting of an ultraviolet curing resin and a thermosetting resin. In this example, the resins may be an epoxy resin or an acrylic resin.
In the second production method, preferably, a liquid is used as the material of a high refractive index and, after filling into the cavity, ends of the core pattern are sealed. In this example, a liquid of a viscosity of 10 cps or less may be used. For example, an ultraviolet curing resin containing silicone modified oligomer, a polyamide acid solution of fluorinated polyimide, or cyclohexane monomer, or a mixed liquid of benzene and decalin may be used.
In the first and second production methods, preferably, the predetermined core pattern groove is formed in the surfaces of the first and second optical members by means of press molding using a molding die in which the base material mainly contains diamond or diamond-like carbon. Diamond or diamond-like carbon is a material which can realize a die of high mechanical strength and excellent durability. When dry etching is performed by using an etching gas which mainly contains oxygen, diamond or diamond-like carbon can be etched at a high rate, and the resulting worked face is very smooth. Therefore, a pattern which is fine and deep can be correctly formed in the surface of the die.
In the first and second production methods, preferably, section shapes of grooves respectively formed in the first and second optical members are symmetrical about the sticking face.
As described above, in the optical waveguide component of the invention, the core can be formed by means of press molding, and hence the optical waveguide component can be simply produced and easily connected with an optical fiber. Therefore, it is possible to realize a waveguide device which is high in productivity and low in production cost. According to the invention, moreover, not only a passive device but also an active device such as a temperature sensor can be easily realized.